The effect of selenium on the neurotoxicity of methylmercury was studied. It was found that equimolar dose of selenium was very effective against the toxic effects of methylmercury. The survival rate and body growth rate of the mercury-intoxicated animals were significantly improved by selenium administration. Morphological studies by both light and electron microscopies also provided morphological evidence for such protection. The blood glutathione peroxidase activity (GSHx) declined steadily as the intoxication progressed and may serve as a sensitive indicator of methylmercury poisoning. The GSHx was stabilized by selenium treatment but still declined at the end-stage of the poisoning. No significant redistribution of mercury in the nerve cells was observed, indicating that selenium exerts a protective effect without altering the mercury level or distribution in the nervous system.